Diborane is highly soluble in tetrahydrofuran and forms a borane addition compound with it. [J. R. Elliott, W. L. Roth, G. F. Roedel, and E. M. Boldebuck, J. Am. Chem. Soc., 74, 5211 (1952)]. Such solutions are highly valuable for the hydroboration of olefins and the reduction of organic compounds as is illustrated by the following chemical equations: ##STR1## The diborane solutions, unfortunately, are unstable and lose hydride with reductive opening of the tetrahydrofuran ring [J. Kollonitsch, J. Am. Chem. Soc., 83, 1515 (1961)].
A solution of diborane in tetrahydrofuran can be prepared without handling diborane gas by treating a suspension of sodium borohydride in tetrahydrofuran with the stoichiometric amount of boron trifluoride, as is shown by the following chemical equation: ##EQU1## This process is disclosed in my U.S. Pat. No. 3,634,277, with the use of an excess borohydride to stabilize the solution. The resulting diborane solution contained some of the sodium fluoroborate originally present as a finely divided material which was difficult to remove by settling, filtration or centrifugation.
One method which is sometimes used to convert a finely divided material into a more easily handled solid is the application of heat. Elevating the temperature of the suspension, at times, will result in crystal growth or agglomeration of the particles. The resulting larger sized particles are then removed more easily by settling, filtration or centrifugation.
The application of heat to the diborane solution in tetrahydrofuran, such as that produced according to my U.S. Pat. No. 3,634,277, to make the finely divided sodium fluoborate therein more easy to handle was not feasible. This was because raising the solution temperature to reflux resulted in escape of diborane as a volatile gas and greatly magnified reductive cleavage of the solvent.
In my U.S. Pat. No. 3,882,037 I described the synthesis of stabilized borane-tetrahydrofuran solutions. Such solutions, containing an equimolar amount of an organic sulfide, i.e., an aliphatic, alicyclic, or cyclic sulfide, are readily stored for long periods of time at ambient temperatures. These solutions can be prepared also without handling diborane gas by treating suspensions of sodium borohydride in tetrahydrofuran, containing the aliphatic, alicyclic, or cyclic sulfide, with the stoichiometric amount of boron trifluoride. Unfortunately, even in this process the sodium fluorborate separates as a finely divided precipitate which again has proven difficult to remove by settling, filtration or centrifugation.
The stabilized borane-tetrahydrofuran solutions containing an organic sulfide summarized above and disclosed in my U.S. Pat. No. 3,882,037 were studied for extended storage stability at 25.degree. C. (ambient temperature), as see Table I of that patent, but not at a higher temperature.